1. Field of the Invention
The present invention relates to a system for analyzing a digital representation of a signal to determine whether the signal is periodic, i.e., has constant or regularly repeating uniform frequency components, or is essentially random with respect to frequency, and particularly to a system for detecting call progress signals in automated telephone equipment, which are periodic, as compared to voice signals which are of essentially random frequency content within the audible frequency band over any time period of substantial length.
2. Prior Art
In various types of automated "voice processing" telephone equipment it is necessary to distinguish between a voice signal and a call progress signal for orderly and convenient operation of the equipment. For example, in automatic dialing equipment of the type intended to provide a recorded message or conduct a survey or poll without human intervention, the equipment should detect a dial tone before dialing and, after dialing, detect a busy signal, congestion signal or ring back signal transmitted from the local telephone service company or a private branch exchange (PBX). The equipment also must detect when the telephone is answered (placed off-hook) and, for most efficient use, should reliably detect signals from a fax machine or modem and detect when the telephone is hung up (returned to its on-hook condition).
On the receiving end, automated attendant systems which take the place of or supplement the traditional human receptionist also may include equipment for automatically recording a message from the caller. The equipment should reliably detect when the caller has hung up, which usually is followed by a call progress signal, namely, a dial tone or reorder signal. Failure to detect the call progress signal can result in recording long segments of it. In addition to being irritating at the time of playback, such a malfunction also can result in consuming large quantities of computer memory for sophisticated systems in which the voice messages are stored in digital form in a computer. On the other hand, falsely characterizing a voice signal as a call progress signal may result in cutting off the caller before the desired message is completed which is frustrating to the caller.
A complicating factor has been the increase in the use of PBX equipment from different manufacturers. Such equipment lacks uniformity in corresponding types of call progress signals. A busy signal or reorder signal of one system can be different from a busy signal or reorder signal of another system both with respect to its frequency components and to duration of and spacing between the loud sounds.
Known prior systems have concentrated on the pattern of the periods of sound and silence to detect the existence of a call progress signal. Bower et al. in their U.S. Pat. No. 4,201,896, issued May 6, 1980, emphasize that
Conventional methods of ring and busy detection through use of tone decoders [present] problems due to inconsistencies between local telephone companies with respect to signaling frequencies. PA0 To determine that a call has been answered, it is first necessary to determine that at least one ring has appeared on the telephone line. A discrimination that this first ring has appeared on the telephone line is determined by a continuous VOX output of at least 900 milliseconds. PA0 VOX activation of more than 1.2 seconds but less than 50 percent VOX activity for any five second period . . . PA0 If, after one or more rings, the called party answers the phone by lifting receiver and saying "Hello", a normal response, the VOX will be activated by the "Hello" on the line. This word is normally more than 200 milliseconds but less than 1.2 seconds in duration. Thus, a VOX output for more than 200 milliseconds but for less than 1.2 seconds is considered to be an answer. PA0 . . . comparator means 102 . . . connected to a bias supply V+ so that, in effect, the comparator means 102, as also explained in U.S. Pat. No. 4,160,125 and shown in FIG. 2 thereof, acts as a threshold amplifier in that only inputs from the amplifier means 34 are passed thereby if they exceed the bias level V+ applied to the inverting input thereof. PA0 All such inputs which exceed the preset bias or squelch level associated with the comparator means 102 are provided as outputs on the conductor 6 so that, in effect, the comparator 102 only permits signals above a certain level to appear at its output whereupon the zero crossings of an input signal are applied to the input of [an] interrupt request flip-flop means 8 to cause an interrupt request to be generated on [a] conductor 10 thereby. Accordingly, the interrupt request flip-flop means 8 monitors the output of the comparator means 102 and generates an interrupt request output on conductor 10 each time a zero crosspoint is detected on the telephone line being monitored. PA0 The microprocessor means 12 is responsive to each interrupt request generated thereby to treat such requests as data corresponding to the timing of zero crosspoints of the input signal on the telephone line to be monitored. While the precise manner in which the microprocessor means 12 processes this information and makes determinations in accordance with the program routine established in the ROM 14 will be described in great detail in conjunction with FIGS. 2-4; it is here sufficient to appreciate that the microprocessor means 12 makes determinations as to whether or not the information being received corresponds to a periodic signal, a non-periodic signal, or a non-existant signal which corresponds to silence. Thereafter, this information is employed to ascertain whether or not (1) a busy signal is present, (2) a recorded message is present, (3) a ring signal is present or (4) an answered telephone is on the line. PA0 all that the apparatus is concerned with from this point out is the duration of high and low intervals corresponding to the presence and absence of a signal within a pass band.
Column 4: lines 40-44. That patent proposes that a voice-activated device (VOX) be used to determine if the "average energy level" is above a preset bias (see the paragraph beginning at column 5, line 39).
Column 6, lines 43-48. Subsequent rings are determined by
Column 7, lines 21-23. Thereafter,
Column 6, lines 51-57.
Additionally, in the Bower et al. system silence for more than eight seconds is considered to be an answer (see column 6, lines 58-60). As described toward the top of column 7, a busy signal is considered to be present if there are "8 VOX activations of more than 400 milliseconds but less than 900 milliseconds". Characterization of the received signals is based almost exclusively on the durations of the bursts of detected signal energy.
The system of Smith U.S. Pat. No. 4,356,348, issued Oct. 26, 1982, incorporates VOX circuitry (disclosed in commonly assigned Bower et al. U.S. Pat. No. 4,160,125) which includes
Column 3, lines 58-64. The Smith system measures the intervals between "zero crossings" as compared to the bias voltage V+. As explained in the paragraph beginning at column 6, line 19,
The "interrupt request" is fed to a microprocessor.
Column 6, lines 35-52. In general, the microprocessor determines the "most prevalent interval" between "zero crossings" which is compared with "succeeding time intervals" between "zero crossings" (see the SUMMARY OF THE INVENTION beginning at column 2, line 30) so that signals are characterized based on the timing pattern of bursts of input signals having energy above a preset level.
Cave et al. U.S. Pat. No. 4,405,833, issued Sept. 20, 1983, discloses another system which concentrates on the pattern of silence and sound signals, although Cave et al. measure "envelopes" of dual tone signals within a selected frequency band. The detected pattern is compared to "expected" patterns of call progress signals.
Szlam et al. U.S. Pat. No. 4,477,698, issued Oct. 16, 1984, discloses another type of automated telephone equipment having a call progress signal detection system concentrating on the duration and timing of input signal frequency components within a certain frequency range (100 hertz to 750 hertz). As stated in the paragraph beginning at the top of column 9, after it has been determined that a signal has frequency components within the pass band of a filter,
There can be problems with systems of the type described above because they concentrate only on the duty cycle of a signal--the repetitive pattern of sound and silence--although systems such as those disclosed in the Cave et al. patent and the Szlam et al. patent are somewhat more sophisticated by examining only frequencies within a certain pass band or having a certain relationship. Errors in discrimination between voice and call progress signals have been undesirably frequent. Long loud background noises may be characterized as dial tone such that a calling or called party is cut off by the automated equipment. Some systems have trouble detecting that a called telephone has been answered if the answering occurs before the first ring back signal is generated. Without reliable detection of call progress signals, the usefulness or at least popularity of voice processing systems will be more limited than it otherwise would be.